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Journal of Resources and Ecology >

2022 , Vol. 13 >Issue 1: 34 - 40

DOI: https://doi.org/10.5814/j.issn.1674-764x.2022.01.004

Ecosystems in Response to Global Change

Leaf Longevity in a Timberline Tree Species Juniperus saltuaria in the Sergymla Mountains, Southeastern Tibet

  • ZHANG Lin , 1, 2, * ,
  • YANG Liu 1, 3 ,
  • GUO Ying 1, 3 ,
  • SHEN Wei 1 ,
  • CUI Guangshuai 1, 3
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  • 1. State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China
  • 2. Muoto Observation and Research Center for Earth Landscape and Earth System, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China
  • 3. University of Chinese Academy of Sciences, Beijing 100049, China
* ZHANG Lin, E-mail:

Received date: 2021-07-29

  Accepted date: 2021-10-13

  Online published: 2022-01-08

Supported by

The Key Scientific and Technological Research Projects in Tibet Autonomous Region(XZ202101ZY0005G)

The Second Tibetan Plateau Scientific Expedition and Research (STEP) Program(2019QZKK0301-1)

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Abstract

Leaf longevity is an important adaptive strategy that allows plants to maximize photosynthetic carbon gain. Due to the difficulty of identifying overwintering bud scars and distinguishing the age sequence of twigs, leaf longevity is rarely studied in Cupressaceae species, which further limits our understanding of the leaf economic spectrum (LES) for these populations. Here, we investigated the leaf longevity, as well as mass-based leaf nitrogen concentration (Nmass), of Juniperus saltuaria at different canopy heights for both subalpine and alpine timberline forests in the Sergymla Mountains, southeastern Tibet. We found that the mean leaf longevity was 4.2±1.2 years, and overall it did not differ significantly between different elevations. Along the vertical profiles of juniper canopies, the leaf longevity did not reflect a linear trend. With increasing leaf longevity, Nmass showed declining trends. We further analyzed the relationship between leaf longevity and the corresponding length of green twigs, and found that the length of green twigs could only explain 1%-3% of the variation in leaf longevity, indicating that the length of green twigs is a poor predictor for the variation in leaf longevity. In summary, for the J. saltuaria species in timberline or nearby subalpine forests, the effects of elevation and canopy depths on leaf longevity are minor, and the leaf trait analysis is in accordance with the prediction of LES.

Key words： canopy height; elevation; leaf nitrogen concentration; length of green twigs; Cupressaceae

Cite this article

ZHANG Lin , YANG Liu , GUO Ying , SHEN Wei , CUI Guangshuai . Leaf Longevity in a Timberline Tree Species Juniperus saltuaria in the Sergymla Mountains, Southeastern Tibet[J]. Journal of Resources and Ecology, 2022 , 13(1) : 34 -40 . DOI: 10.5814/j.issn.1674-764x.2022.01.004

1 Introduction

Leaf longevity, the time period during which an individual leaf or part of a leaf is alive (Pérez-Harguindeguy et al., 2013), is an important adaptive strategy that allows plants to maximize photosynthetic carbon gain (Field, 1983; Kikuzawa, 1995; Reich et al., 1997). Plants with greater leaf longevity usually occur in cold or nutrient-limited environments, while those with shorter leaf longevity occupy warm and nutrient-rich environments. The leaf economic spectrum (LES) illustrates that species at the quick-return end generally have high leaf nutrient concentrations, high photosyn thetic capacity and short leaf duration, whereas, those at the slow-return end tend to have low nutrient concentrations, low rates of photosynthesis and long leaf lifetimes (Wright et al., 2004). In stressful environments, evergreen plants typically adopt strategies near the ends of the spectrum rather than those in between, therefore exhibiting relatively longer leaf longevity.
The methods for the estimation of leaf longevity vary according to different plant functional types or species. For example, at an individual or branch level, the characteristic of leaf cohorts for conifer species from Picea, Abies, and Pinus are usually applied to distinguish between different-aged leaves (Gower et al., 1993; Luo et al., 2005; Zhang et al., 2010; Pérez-Harguindeguy et al., 2013), while leaf scars are regarded as a key boundary between new and old leaves in evergreen broadleaved species (Wang et al., 2000; Zhang et al., 2010). However, due to the difficulty in distinguishing new leaves from the old ones, leaf longevity measurements for species in Cupressaceae are very limited (Harlow et al., 2005), which largely constrains our ability to test or apply LES to the Cupressaceae species.
Compared with the limited studies on the leaf traits of Cupressaceae species, the distribution area for Cupressaceae species is very wide across Tibet. For example, one of the most widely-distributed Cupressaceae species, Juniperus pingii var. wilsonii, dominates above alpine timberlines in southeastern Tibet, and has become a predominant woody species in alpine shrublands in western and central Tibet. It also forms typical shrublines in Saga County in west Tibet, which is regarded as the highest shrubline (5280 m) in the world (Lu et al., 2019). Besides, alpine timberlines dominated by Juniperus species (J. saltuaria, J. tibetica), with elevations ranging from 4300 to 4900 m, are among the highest timberlines in the world (Miehe et al., 2007).
In this study, we focused on J. saltuaria timberline trees in southeastern Tibet because it represents the upper limit of tree survival, and because the cold environment and the very short growing season length (around 100 days per year) may exert strong impacts on tree growth. Because of the obstruction by high mountains and the inconvenience of transportation, this area suffers from relatively minimal human disturbance, and provides an ideal natural laboratory for studying leaf longevity and its possible limiting factors at the timberline ecotone. Through an investigation on the leaf longevity of Juniperus saltuaria at different canopy heights for both subalpine and alpine timberline forests, we aim to: 1) Explore how long the evergreen leaves may persist in such a stressful environment; 2) Determine whether the leaf longevity differs between elevations or canopy heights. Since the green leaves in Juniperus species are closely attached to lignified stems forming a green twig, and the characteristics of leaf cohorts may in some way resemble those from other conifer species like Abies and Picea, we assume that the length of green twigs might be a good predictor of leaf longevity. We therefore measured the lengths of green twigs while counting the corresponding number of rings, i.e., leaf longevity, to test this assumption.

2 Data and methods

2.1 Study area and species

The Sergyemla Mountains, part of the Nianqing-Tanggula Mountains, are located in the northwestern part of the Great Canyon, Yarlung Zangbo, southeastern Tibet. This study was conducted on a south-facing slope near the peak of the Sergymla Mountains (29°36′N, 94°36′E) (Fig. 1a). According to 12-year (2006-2017) climatic measurements on the same slope at 4425 m, the annual, January, and July mean temperatures are 1.1 ℃, -7.1 ℃, and 8.3 ℃, respectively. Annual precipitation is 950 mm, and it mostly falls during June and September. The main soil type is acid brown soils. Juniperus saltuaria is the typical timberline species on south-facing slopes, and generally forms relatively open subalpine forests (Fig. 1b, c). The shrub layer under J. saltuaria forests is mainly composed of Rhododendron aganniphum var. schizopeplum, Salix oritrepha, and R. bulu. Common herb species include Primula calderiana, P. atrodentata, and Rhodiola fastigiate, among others.
Fig. 1 (a) A map of the study area and sampling plot; (b-d) The physiognomy and an individual of Juniperus saltuaria in the Sergymla Mountains.

2.2 Sampling methods and leaf trait measurements

Along the slope, two sites with different elevations were selected for leaf longevity measurements. One is at the timberline forest with an altitude of around 4420 m, and the other is in a subalpine forest with an altitude of 4290 m (Fig. 1a). For each site, three 20 m×20 m plots were set to investigate the tree height and the diameter at breast height (DBH), and the mean height and DBH were calculated (Table 1), which were then regarded as the criteria for the selection of standard individuals. At each site, 5 individuals with the mean tree height and/or DBH were selected for leaf sample collection. For each sampled individual, we collected one branch with a length of 20-30 cm at the upper, middle, and lower canopy heights by using a pole. All samples were tagged and stored in a refrigerator at the southeastern Tibetan plateau station for integrated observations and research.
Table 1 Stand characteristics for Juniperus saltuaria forests at the two elevations
Forest type Elevation (m) Stand age (yr) Mean tree height (m) Mean DBH (cm) Basal area (m2 ha-1) Tree density (trees ha-1)
Timberline forest 4425 300-400 6.0 13.3 39.8 2050
Subalpine forest 4290 400-500 9.5 26.8 60.7 708
For each sampled branch, 8-12 intact branchlets (subsamples) with clear boundaries between the green and brown leaves/branches were randomly selected for leaf longevity measurements. All the samples were recut at the boundary with a razor blade and the number of rings of the section were counted with the aid of an optical microscope (10×10). Also, the lengths of green twigs were measured with a digital vernier caliper. In all, 177 and 129 branchlets were selected for counting the rings among the timberline and subalpine Juniper trees, respectively.
After ring-counting, all the green twigs for each sampling branch at a given canopy position were combined and grounded to a fine power after drying at 70 ℃ for 48 hours. The mass-based leaf nitrogen concentration (Nmass) was then analyzed with a micro-Kjeldahl assay. We only measured Nmass for the upper and lower branches since the samples for the middle canopy positions were unexpectedly discarded after the tree-ring observations.

2.3 Statistical analysis

The one-way ANOVA was used to test the differences in leaf longevity between different canopy heights or elevations. The relationships between Nmass and leaf longevity were explored by linear regression analysis. Before analyzing the relationship between the length of green twigs and corresponding leaf longevity, the data in both groups were LN-transformed. The differences in the slopes of the relationships between Nmass and leaf longevity were further tested using analysis of covariance (ANCOVA). All statistical analyses were performed with SPSS 19 for Windows (SPSS Inc., Chicago, USA).

3 Results and analysis

3.1 Variations in leaf longevity with elevation and canopy height

At the branch level, the leaf longevity of Juniperus saltuaria varied substantially from about 1 to 8 years, with an average value of 4.2 years. Most sections yielded a value of 3-5 years (Fig. 2, 79.84% for the subalpine site, 78.53% for the timberline site). Overall, no significant difference in leaf longevity existed between the different elevations (Fig. 3), i.e., the leaf longevity for the timberline forest (4.0 years) was comparable with that of the subalpine forest (4.4 years). The leaf longevity did not differ significantly between canopy heights at the timberline site, while it seemed a bit higher at the middle canopy position in the subalpine forest compared with the upper or lower positions (Fig. 3).
Fig. 2 Normal distribution in leaf longevity for Juniperus saltuaria at different elevations in the Sergymla Mountains
Fig. 3 Variations in leaf longevity among canopy depths for Juniperus saltuaria at different elevations in the Sergymla Mountains

Note: Different upper-case letters indicate significant differences among different canopy depths, while different lower-case letters indicate significant differences between different elevations.

3.2 Relationship between leaf longevity and nitrogen concentration

Nmass tended to decrease with increasing leaf longevity in the timberline forest (Fig. 4, R2=0.67, P<0.05), though the relationship in the subalpine forest was not statistically significant (Fig. 4, R2=0.21, P>0.05). Since the slopes of the two trend lines did not differ from each other (-0.76 vs -0.82), the data for the two sites were pooled together, and this combined Nmass showed a significant negative correlation with leaf longevity (R2=0.31, P<0.05).
Fig. 4 Relationship between leaf longevity and mass-based leaf nitrogen content (Nmass) for Juniperus saltuaria at different elevations in the Sergymla Mountains

3.3 Variations in leaf longevity with length of green twigs

For the subalpine forest site (4290 m), the leaf longevity was not related with the length of green twigs (Fig. 5, R2=0.01, P>0.05). While a significant positive relationship was detected for the timberline site (4425 m), the explanatory power was quite low (Fig. 5, R2=0.03, P<0.05).
Fig. 5 Variations in leaf longevity with length of green shoots for Juniperus saltuaria at different elevations in the Sergymla Mountains

4 Discussion

The leaf longevity of some evergreen species, especially those in Cupressaceae, are seldom reported due to the difficulty in identifying overwintering bud scale scars and distinguishing the age sequence of twigs (Smith et al., 2019). In this study, the leaf longevity of our target species, Juniperus saltuaria, was 4.2 years, which is within the range of the published results for various Cupressaceae species (Table 2). Specifically, this value is higher than those reported for J. scopulorum in coastal Washington State (2.5 years, Pease, 1917) and J. thurifera in France and Spain (2.46 years, Montesinos et al., 2010), indicating the relatively stressful environment in our study area at or near alpine timberline ecotone where J. saltuaria dominates. Compared with other genera in Cupressaceae, the leaf longevity for Juniperus is relatively low. For example, the leaf longevity of western red cedar (Thuja plicata) in the Rocky Mountains and hinoki cypress (Chamaecyparis obtuse) in southwestern Japan reached 8.9 and 6.3 years (Table 3, Harlow et al., 2005; Miyamoto et al., 2013), respectively. Some variations may be due to the relatively warmer conditions on the south-facing slope (Liu and Luo, 2011), as the leaf longevity of J. saltuaria is significantly lower than that of Abies georgei var. smithii (about 10 years) at the timberline ecotone on the opposite slope.
Table 2 Comparisons in leaf longevity for some of Cupressaceae species
No. Species Location Leaf longevity (range)(yr) Reference
1 Juniperus saltuaria Linzhi, Tibet, China 4.2 (1-8) This study
2 Juniperus scopulorum Coastal Washington State, USA 2.5 (1-4) Pease (1917)
3 Juniperus thurifera France/Spain 2.46 (2.0-3.2) Montesinos et al. (2010)
4 Juniperus monosperma New Mexico, USA 6.5 Wright et al. (2004)
5 Chamaecyparis obtusa Karakawa, Japan 3.9 (2.7-5.9) Miyamoto et al. (2013)
6 Chamaecyparis obtusa Okuono, Japan 6.3 (4.6-7.8) Miyamoto et al. (2013)
7 Thuja plicata Coastal Washington State, USA 3.5 (1-7) Pease (1917)
8 Thuja plicata Northern Idaho, USA 8.9 (6.8-10.6) Harlow et al. (2005)
9 Thuja occidentalis Wisconsin, USA 4 Wright et al. (2004)
Mean 4.7
The most commonly used methods for obtaining leaf longevity are either to track the birth and death of individual leaves at repeated census intervals, or to count the leaves which are produced and have died over a time interval (Pérez-Harguindeguy et al., 2013), although this method is seldom applicable for species with long-lived leaves like those from Abies and Picea, whose leaf longevity can be obtained by counting the number of cohorts (Zhang et al., 2010; Pérez-Harguindeguy et al., 2013). However, for some species in Cupressaceae, the above methods are not practical due to the difficulty in distinguishing leaves from twigs. Instead, counting the number of rings of green twigs provides the leaf age or longevity estimation for these species. This technique is also used to test the results of counting cohorts for conifers (Gower et al., 1993), especially when leaf longevity is quite long.
The LES has been tested by many case studies since 2004 (e.g., He et al., 2006; Yu et al., 2014). However, until now available data for Cupressaceae species are scarce. The negative relationship between leaf longevity and Nmass found in this study indicates that the green twigs with higher leaf longevity possess lower photosynthesis, while those with lower leaf longevity reflect faster carbon sequestration since Nmass is highly correlated with net photosynthetic rate (Field and Mooney, 1986). This relationship is in accordance with the general view of the LES.
It is well known that the light environment varies along the vertical profile of a forest canopy, especially within a dense forest. Therefore, the stronger radiation at the upper crowns generally corresponds with higher photosynthesis and yields shorter leaf longevity, while the shaded environment at the deepest crowns probably yields the longest leaf longevity (Ellsworth and Reich, 1993; Seiwa, 1999; Zhan and Yang, 2012). In this study, however, we did not find that the leaf longevity varied with canopy height. The possible reasons may lie in the following characteristics of our species. First, unlike the larger crown diameter for Abies georgei var. smithii on the opposite north-facing slope, the Juniperus trees possess a smaller crown diameter and form discontinuous open forest canopies. Second, Juniperus trees are specifically distributed on the south-facing slopes where the light environment is relatively even, compared with the north-facing slopes. Third, the heights of Juniperus trees are less than 10 m in subalpine forest and decline to 6 to 7 m at the timberline ecotone due to the low-temperature-induced water limit (Liu et al., 2016). These features all lead to the relatively even light environment along the canopy profiles, which largely mediates the vertical pattern of leaf longevity.
For conifer species in cold areas, an annual cohort is born each year, forming a significant leaf age sequence according to the annual scars between each cohort. Therefore, it seems that the lengths of the cohorts may be used to infer the maximum leaf age, i.e., leaf longevity, although this has never been tested for Cupressaceae species. However, contrary to our hypothesis, the longer green stem did not necessarily yield a longer leaf longevity, or the explanatory power might be very low. This may be related to the auxin levels and/or the delivery rate of cytokinin along the vertical light profiles (Tworkoski et al., 2006; Boonman and Pons, 2007). In other words, probably due to the differences in the auxin and/or cytokinin concentrations, the green stems in the upper canopies are usually longer than those in the lower ones, while the leaf longevity did not differ among the different canopy depths. We therefore argue that the length of green stems is a poor predictor of leaf longevity for Juniperus saltuaria.

5 Conclusions

Juniperus saltuaria, as one of the highest timberline species in the world, showed a mean leaf longevity of 4.2 years in the Sergymla Mountains, southeastern Tibet. Possibly due to the relatively open canopy and the shorter elevation distance between the two sites, the effects of canopy depth and elevation on leaf longevity in this study seem minor. The negative relationship between leaf longevity and nitrogen concentration indicates that the leaf trait relationship for this Cupressaceae species is in accordance with the prediction of LES. However, contrary to our hypothesis, the length of green twigs seems to be a poor predictor for the variation of leaf longevity. Therefore, care should be taken when employing the length of twigs as an indicator of leaf longevity for Cupressaceae species. Also, further studies on measurements of leaf photosynthesis and related nutrients (phosphorus, potassium) in the future are needed to elucidate the whole LES.

We are grateful to the staff in Southeast Tibet Observation and Research Station for the Alpine Environment of CAS for the field work.

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